They also found more killer T cells - which help patients fight the infection - in the nose and throat of those with the protective variant.

The IFITM3 gene produces an anti-viral protein that helps block flu in lung cells and boosts killer T cells that clear infection in the airways.

An earlier study of Chinese patients by another team found a link between another IFITM3 variant (rs12252) and flu severity.

But the underlying mechanism has remained unclear and the rs12252 mutation is rare in individuals of European ancestry.

So Dr Thomas and colleagues looked for other IFITM3 mutations in US patients and found one known as rs34481144.

Those carriers were likely to become infected with flu more rapidly and to develop more severe symptoms.

High-risk variant

Checks on flu infections in 86 children and adults in Memphis found two-thirds of those with the most severe symptoms carried at least one copy of the high-risk IFITM3 variant - compared to only 32 percent of those with milder symptoms.

It was also linked to severe and fatal flu infections in 265 critically ill paediatric patients who did not have any other health problems. Of the 17 who died 14 carried at least one copy of the mutation.

Dr Thomas said: “When we looked at patients of European descent who died they all carried at least one copy of the high-risk variant.”

It’s found in the region of IFITM3 linked to a master regulator called CTCF which can suppress gene activity.

Dr Thomas said: “While this research focused on flu infections, the mechanism we identified has implications for regulating many genes involved in anti-viral activity.

“CTCF has gained prominence in recent years as a master regulator of genomic organisation.

“Evidence in this study suggests the high-risk variant we identified may be part of a larger network of CTCF binding sites involved in regulation in other genes with anti-viral activity.”

Flu epidemics can kill thousands or even millions of people. The 1918 flu epidemic is estimated to have affected half the world’s population - and killed 40-50 million people worldwide.

In the UK the annual flu season runs from about October to March or April. Most cases of flu occur between December and February.

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